Determining the exact location of a receiver (e.g. a mobile phone) in an environment can be quite challenging, especially when the receiver is located in an urban environment, or is located within a building. Imprecise estimates of the receiver's position may have “life or death” consequences for the user. For example, an imprecise estimate of a receiver's position, such as the position of a mobile phone operated by a user calling 911, can delay emergency personnel response times.
Positioning systems used to estimate the position of the receiver, like the Global Positioning System (GPS), have been in use for many years. Unfortunately, poor signal conditions found in urban and indoor environments may degrade the performance of these conventional positioning systems. To improve positioning accuracy in urban and indoor environments, GPS may be augmented by positioning systems that use terrestrial transmitters. Even augmented positioning systems can be ineffective at estimating a receiver's position in an urban area or inside a building. Thus, new approaches that accurately determine the location of a receiver, or that refine initial estimates of the receiver's position are needed.